1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a facsimile machine, and a printer, in which a toner is fixed on a recording medium by application of heat.
2. Description of the Background
Image forming apparatuses, such as copiers, facsimiles, and printers, which form toner images on recording media are widely used. In such an image forming apparatus, toner is heated and softened to be fixed on a recording medium. To reliably fix the toner on the recording medium, the toner is required to soften sufficiently. Because the toner needs a large amount of heat to sufficiently soften, a large amount of electric power is consumed, which is against recent momentum of energy conservation. A proportion of electric power consumption in fixing toner on recording media to total electric power consumption in an image forming apparatus is relatively high. Therefore, it is desirable that electric power consumption is reduced as much as possible. There have been various attempts to reliably fix toner on recoding media.
For example, Japanese Patent No. 4224076 proposes applying a fixing liquid that softens and/or swells toner to a toner image on a recording medium. This technique meets energy conservation because no heat is required. Japanese Patent No. 4354164 also proposes a technique requiring no heat which applies a fixing liquid to a toner image on an intermediate transfer member so that the toner image is transformed into an adhesive film and makes the film-like toner image be fixed on a recording medium owing to its adhesiveness.
The above-described techniques have a problem that the toner image is disturbed upon application of the fixing liquid. To solve this problem, in Japanese Patent No. 4224076, the fixing liquid is applied to toner images on a recording medium by a non-contact ink jet method. The application amount of the fixing liquid is gradually increased along the direction of feed of the recording medium. However, it is likely that the toner powders scatter and contaminate nozzles of the ink jet head upon reception of the injected fixing liquid, even when the application amount of the fixing liquid is small. As a result, the nozzles may be clogged with the toner powders. In order to reliably fix a toner image on a recording medium, a relatively large amount of the fixing liquid is needed so that the fixing liquid reliably reaches the contact point of the toner image and the recording medium. However, such a large amount of the fixing liquid requires a large amount of heat when dried, resulting in undesired increase in energy consumption and drying time.
In Japanese Patent No. 4354164, as described above, a toner image on an intermediate transfer member is transformed into a toner film upon application of the fixing liquid at environmental temperatures. This technique has a problem that the fixing liquid is likely to contaminate image forming parts and undesirably forms toner films on the image forming parts at environmental temperatures.
Japanese Patent Application Publication No. 2007-121652 describes an image forming apparatus employing a wet developing method using a liquid developer comprising a toner and a carrier liquid. In this image forming apparatus, a fixing liquid, which is compatible with the carrier liquid, is previously applied to a recording medium and a toner layer is formed on the recording medium by the wet developing method. The carrier liquid existing between the toner particles in the toner layer is flowed so that the toner layer is dissolved and swelled to be fixed on the recording medium. This technique prevents image disturbance upon application of the fixing liquid, but causes image disturbance upon interaction between the fixing liquid and the carrier liquid. This technique also has a problem that the fixing liquid is likely to contaminate image forming parts and undesirably forms toner films on the image forming parts at environmental temperatures. It may take an unnecessarily longer time when the fixing liquid is applied before an image is transferred onto the recording medium compared to when the fixing liquid is applied after the image is transferred onto the recording medium. The fixing liquid compatible with the liquid developer may not be applicable to other images formed without the liquid developer.
Image recording methods which inject toner toward a recording medium, such as toner jet, direct toning, and toner projection, have been proposed in, for example, Japanese Patent Application Publication No. 2009-39977. In these methods, a liquid is applied to a recording medium before a toner image is formed thereon. Therefore, the occurrence of image disturbance due to application of liquid can be prevented. However, it requires a large amount of heat to dry the liquid, resulting in high electric power consumption and long fixing time. The liquid applied to the recording medium includes a softener that dissolves or swells resins included in toner. Thus, the recording medium to which the liquid is applied prevents toner from scattering and accelerates fixation of the toner. This is because the recording medium to which the liquid is applied has a higher adhesive force to toner and a lower repulsive force to toner. Specifically, the liquid is absorbed in fibers of the recording medium. Such fibers absorbing the liquid are soft enough to prevent toner from scattering. To further dissolve or swell the toner to accelerate fixation of the toner, the liquid needs to reliably contact the toner. When the injected toner reaches and adheres to the recording medium, only a slight amount of the liquid existing at the surface of the recording medium can contact the toner and most of the liquid cannot penetrate the toner layer. In this case, the toner may scatter and may not be reliably fixed on the recording medium. In a case in which the liquid is previously applied to an intermediate transfer member before a toner image is formed thereon, penetration of the liquid into the toner image may be accelerated. However, the liquid may adversely affect image forming members. In particular, surface conditions of the resulting images can be optically detected with various apparatuses disclosed in, for example, Japanese Patent Application Publication Nos. H10-49654, 2000-11493, and 2008-32675.
However, such liquids that reduce electric power consumption in fixing cause problems of, for example, image disturbance, increase in electric power consumption in drying, lengthening of fixing duration time, and contamination of image forming members, and these problems have not been solved.